Tests that oopif iframes are rendered inline.


Dump tree
- <html>
      <head></head>
    - <body>
        - <div>
            - <iframe id="page-iframe" src="http://127.0.0.1:8000/devtools/oopif/resources/inner-iframe.html">
                - #document
                    - <html>
                          <head></head>
                          <body>IFRAME\n</body>
                      </html>
              </iframe>
          </div>
      </body>
  </html>

Dump tree
- <html>
      <head></head>
    - <body>
        - <div>
            - <iframe id="page-iframe" src="http://devtools.oopif.test:8000/devtools/oopif/resources/inner-iframe.html">
                - #document
                    - <html>
                          <head></head>
                          <body>IFRAME\n</body>
                      </html>
              </iframe>
          </div>
      </body>
  </html>